(1) Field of the Invention
The present invention relates generally to axial fans, and more particularly to a flow reversal system for an axial fan that quickly reverses the flow of fluid through the fan without changing the rotational direction or speed of the fan.
(2) Description of the Prior Art
Fan arrangements in conventional air handling systems utilize motor driven fan wheels that draw ventilation air from the outdoors through ducts, louvers and dampers. Although these fan systems may be able to vary their speed and capacity, they almost always have significant amounts of rotational inertia in the fan wheel/motor assembly that can require minutes for the fan wheel to completely stop rotating after the motor has been turned off. However, in situations where air contaminants are detected, it is desirable to shut down the air flow as soon as possible and even reverse same to purge the contaminated air from an indoor environment.
Typically, when contaminants are detected, fan motors are shut off and dampers are used to shut off an air flow in a duct. The use of dampers to rapidly seal off air flow is problematic since most fast-acting damper assemblies are not capable of a completely tight seal. Further, damper seals experience very high pressure excursions when a fan discharge is rapidly closed off. These factors make it extremely difficult to devise a scheme that quickly stops the flow of outdoor air being drawn into an air handling system upon detection of contaminants in that air stream.